The present invention relates to a femoral implant of a knee prosthesis, comprising two condyles integral with a trochlea and a centro-medullary stem adapted to be fixed on a support base arranged between the condyles.
It is known that the centro-medullary stem of an implant must be oriented correctly in the frontal plane in order to respect the patient""s femoral valgus angle, and, on the other hand, in the sagittal plane in order to be well positioned with respect to the inner wall of the trochlea, in order to obtain good primary securing of the implant, the stem having to be suitably placed with respect to the anterior cortex.
The femoral valgus angle is the angle located in the frontal plane between the anatomical axis of the femur and its mechanical taxis, this angle having a value generally between 3 and 11 degrees.
In summary, the centro-medullary stem must be oriented angularly so as to be able to satisfy three imperatives: preservation of the anterior cortex, good secondary securing in the centro-medullary canal, and preservation of the femoral valgus angle of the patient""s knee.
In an attempt to attain these objectives, sets of stems have been proposed in particular having various orientations of femoral valgus angles and in the sagittal plane in order to correspond to a large number of anatomical possibilities. Such embodiments require virtually as many different parts as there are anatomical possibilities, therefore a very large number of parts, which renders the assembly very expensive and cumbersome.
Patent U.S. Pat. No. 5,152,796 and Patent EP-A-0 714 645 describe femoral implants comprising an intermediate piece forming a wedge element or xe2x80x9cpre-stemxe2x80x9d, interposed between the support boss or base in one piece with the frontal plane, and presents a lower face inclined with respect to the surface of the base, which makes it possible to adapt it to the valgus angle of the femur, equally well for the right femur of for the left femur by turning through 180 degrees.
Furthermore, this same Patent EP 0 714 645 provides positioning a screw for securing the pre-stem in the immediate proximity of the trochlea, in the axis of the stem, which has the consequence of considerably restricting the femoro-patellar bearing surface for the patella.
In effect, the result of this proximity of the screw for securing the stem with respect to the trochlea is that the femoro-patellar zone of contact of the patella on the trochlea is necessarily interrupted at the level of the recess for passage of the screw. Consequently, when the patella passes beyond the corresponding interruption of its bearing surface, its bearing is reduced to two points instead of a line of contact. The consequence of this is an increase in the stresses on the patella, therefore wear of the polyethylene of the patella, which may be replaced prematurely. In the patent mentioned above, the line of contact is thus replaced by two points of contact after a flexion of only about 65 degrees.
The patent U.S. Pat. No. 5,405,395 discloses a femoral implant equipped with a pre-stem providing only one angular possibility for the centro-medullary stem, so that the surgeon must have a sufficient number of pre-stems available. Moreover, the line of contact of the patella with the femoro-patellar surface extends over an angle of flexion of considerably less than 90 degrees.
It is an object of the invention to propose a femoral implant arranged so as to satisfy the imperatives set forth hereinabove and in particular to provide the patella with an increased femoro-patellar sliding surface, employing a reduced number of parts compared to the embodiments of the prior art, producing a less expensive assembly of orthopaedic material which is more convenient for the surgeons to use.
According to the invention, the femoral implant for knee prosthesis comprises a pre-stem for connection between the base and the stem, and this pre-stem as well as the base are provided with means allowing an appropriate angular positioning of the pre-stem and of the stem, both in a frontal plane in valgus and in a sagittal plane with respect to a frontal reference plane; the implant also comprises means for securing four pre-stems offering a total of eight possibilities, including four possibilities of femoral valgus angles and four possibilities in the sagittal plane with respect to the frontal reference plane, which is the anterior plane parallel to the anterior face of the femur.
The surgeon thus has at his disposal an orthopaedic assembly constituted by a left femoral implant, a right femoral implant with their stems, and a set of a certain number of pre-stems, offering a wide range of possibilities adaptable to virtually all anatomical cases.
Finally, thanks to the offset positioning towards the condyles of the element for securing the pre-stem and the stem, the femoro-patellar zone of bearing of the patella may be considerably increased with respect to the prior art mentioned hereinabove, up to an angle of flexion close to 90 degrees. This results in a reduction in the stresses on the patella and therefore in the wear thereof.